Gas turbines are widely used in commercial operations for power generation. A gas turbine compresses ambient air, mixes fuel with the compressed air, and ignites the mixture to produce high energy combustion gases that flow through a turbine to produce work. The turbine may drive an output shaft connected to a generator to produce electricity which is then supplied to a power grid. The turbine and generator must operate at a relatively constant speed, regardless of the amount of electricity being generated, to produce electricity at a desired frequency.
Gas turbines are typically designed to operate most efficiently at or near the designed base load. However, the power demanded of the gas turbine may often be less than the designed base load. For example, power consumption, and thus demand, may vary over the course of a season and even over the course of a day, with reduced power demand common during nighttime hours. Continuing to operate the gas turbine at its designed base load during low demand periods wastes fuel and generates excessive emissions.
One alternative to operating the gas turbine at base load during low demand periods is to simply shut down the gas turbine and start it back up once the power demand increases. However, starting up and shutting down the gas turbine creates large thermal stresses across many components that lead to increased repairs and maintenance. Moreover, gas turbines are often operated with additional auxiliary equipment in a combined cycle system. For example, a heat recovery steam generator may be connected to the turbine exhaust to recover heat from the exhaust gases to increase the overall efficiency of the gas turbine. Shutting down the gas turbine during low demand periods therefore also requires shutting down the associated auxiliary equipment, further increasing the costs associated with shutting down the gas turbine.
Another solution for operating a gas turbine during low demand periods is to operate the gas turbine under a turndown regime. In existing turndown regimes, the gas turbine still operates at the speed required to produce electricity at the desired frequency, and the flow rate of fuel and air to the combustors is reduced to reduce the amount of combustion gases generated in the combustors, thereby reducing the power produced by the gas turbine. However, the operating range of typical compressors limits the extent to which the air flow may be reduced, thereby limiting the extent to which the fuel flow may be reduced while maintaining the optimum fuel to air ratio. At lower operating levels, one or more nozzles in each combustor are “idled” by securing fuel flow to the idled nozzles. The fueled nozzles continue to mix fuel with the compressed working fluid for combustion, and the idled nozzles simply pass the compressed working fluid through to the combustion chamber without any fuel for combustion. The turndown regime produces sufficient combustion gases to operate the turbine and generator at the required speed to produce electricity with the desired frequency, and the idled nozzles reduce the fuel consumption. When the power demand increases, fuel flow may be restored to all nozzles to allow the gas turbine to operate again at the designed base load.
Existing turndown regimes are limited in the amount of power reduction that can be achieved. For example, the compressed working fluid passing through the idled nozzles in a turndown regime mixes with the combustion gases from the fueled nozzles and tends to prematurely quench the fuel combustion in the combustion chamber. The incomplete combustion of fuel generates increased CO emissions that may exceed emissions limits. As a result, the minimum operating level during existing turndown regimes may need to be as high as 40-50% design base load to comply with emissions limits for CO and NOx.